PROCESS AND APPARATUS FOR RESIN IMPREGNATION OF A POROUS WEB

Abstract: A process and an apparatus are disclosed for impregnating a porous web with a thermosettable resin. The process involves the use of resin application means comprising a rotating roller (4) on which is deposited a liquid-form thermosettable resin in essentially uncured state, passing a porous web (2) in countercurrent contact with the thermosettable resin so as to transfer the thermosettable resin into the porous web, exposing the resin-containing porous substrate to an elevated temperature lower than that maintained for partial cure; and passing the resin-containing fibrous web to a heating zone to partially cure the resin and form a prepreg. The process is particularly suited for application of a solventless resin formulation to a glass web in the preparation of a prepreg for an electrical laminate. The apparatus includes resin application means comprising a rotating roller (4); means for applying a liquid-form thermosettable resin onto the rotatable roller; means for advancing, in a countercurrent direction with respect to the direction of motion of the roller, a fibrous web (2) to the roller and in contact with the thermosettable resin thereon and thence to a resin cure zone; and means (13) between the rotatable roller and the resin cure zone (14) to apply heat to the resin-containing substrate.

Description: PROCESS AND APPARATUS FOR RESIN IMPREGNATION OF A POROUS EB This invention relates to the impregnation of a porous web with a thermosettable resin. In a specific embodiment, the invention relates to a process and apparatus for impregnation of a fibrous glass substrate with a solventless thermosettable resin system. The manufacture of the cured thermosettable resin base of an electronic circuit board begins with the impregnation of a fibrous glass substrate with a liquid thermosettable resin system. The resin-impregnated substrate is then partially cured to form a "prepreg". A set of layered prepregs is heated under pressure to fully cure the resin and to form a hard laminate, which serves as the base for electric circuitry. Although there exist thermosettable resins, such as low molecular weight epoxy resins, which are liquid at room temperature, current circuit board requirements make it necessary to use high-performance resins systems which are solids or viscous liquids at room temperature and to apply the resins to the substrate in melt or solution form. Processing thermosettable resins in the melt, however, is difficult because the high temperatures necessary to melt the resin cause the resin to cure prematurely, resulting in poor "wet-out", or saturation of the substrate by the resin. Current commercial processes for preparing prepregs apply the resin to the substrate using an organic solution of the resin. Solution processes must include a step, usually carried out in conjunction with partial curing of the resin, in which the solvent is removed from the prepreg by heating the solvent to its volatilization temperature. Such a process has a number of disadvantages: First, it requires the disposal or discharge of the organic volatiles. Second, volatilization of the solvent from the uncured resin can result in the presence of voids and irregularities in the prepreg and in the cured laminate. Furthermore, a considerable amount of time is required for the solvent removal step. A process has been proposed for application of a solventless resin to a glass web which involves depositing the solventless resin onto a rotating roller and passing the glass web in countercurrent contact with the deposited resin so as to transfer the resin to the web, which is then passed, via an optional second application roller for application of resin to the opposite side of the web, to a heating zone for partial cure of the resin. This process has been described in European patent No. 476,752. It would be desirable to eliminate the second application roller without compromising the appearance and quality of the product. It is therefore an object of the invention to provide a process and apparatus for impregnating a fibrous substrate with a thermosettable resin. In a specific aspect, it is an object of the invention to provide a simpler process and less expensive apparatus for impregnation of a fibrous glass substrate with a solventless thermosettable resin system. The present invention, therefore, relates to a process for impregnating a porous web with a thermosettable resin by depositing uncured liquid-form thermosettable resin onto a rotating roller, passing the porous web in countercurrent contact with said resin on said roller so as to transfer the thermosettable resin to the porous substrate, and passing the resin-containing porous substrate to a heated cure zone for partial cure of the resin, in which process the resin-containing porous substrate prior to passage to the cure zone is exposed to an elevated temperature greater than 100 °C and less than the highest temperature maintained in said cure zone. In a typical example of the invention, a process is provided for impregnating a porous web with a thermosettable resin, the process comprising: providing resin application means comprising a rotating roller; depositing onto the rotating roller a liquid- form thermosettable resin formulation comprising an essentially uncured thermosettable resin; passing a porous web in countercurrent contact with the thermosettable resin formulation on the rotating roller so as to transfer the thermosettable resin formulation to the porous substrate; exposing the resin- containing fibrous substrate to an elevated temperature of at least 100 °C and less than the highest temperature maintained in the cure zone; and passing the resin-containing fibrous substrate to a cure zone to partially cure the resin and form a prepreg comprising the fibrous substrate and the partially-cured thermosettable resin. Further according to the invention, an apparatus is provided for preparing a prepreg by impregnating a porous web with a thermosettable resin, the apparatus including resin application means comprising a rotatable roller; means for depositing a liquid-form thermosettable resin onto the rotatable roller; means for advancing, in a countercurrent direction with respect to the direction of movement of the rotatable roller, a porous web to the rotatable roller and in contact with the thermosettable resin thereon and thence to a resin cure zone; and heating means between the rotatable roller and the resin cure zone. The invention process provides a relatively simple and inexpensive technique for impregnating a fibrous substrate with a liquid-form thermosettable resin. The process is particularly suitable for impregnating a glass web with a solventless resin system in the preparation of a prepreg for ultimate use in electrical laminates. The object of the resin application process of the invention is to achieve thorough wet-out of the substrate without the use of a second application roller to apply resin to the opposite side of the substrate and to thereby permit a more economical fabrication of a high-quality cured laminate. In the resin application process according to the invention, a fibrous substrate is impregnated with a liquid-form thermosettable resin. Although the invention process can be practiced with solvent-borne resins, the preferred resin system for the invention process is one which does not contain an organic solvent, such as a water-borne resin system or a solventless resin system. For a solventless resin system, the liquid form can be achieved by use of a thermosettable resin which is a low-viscosity liquid at room temperature or which has been heated to a temperature effective to achieve sufficiently low viscosity for thorough wetout of the substrate. In the latter case, of course, the resin system (the thermosettable resin and any curing compounds used therewith) must not cure to any substantial degree at its melting temperature over the length of time of the substrate impregnation process. The invention resin application process and apparatus can be described by reference to Figure 1. The substrate 2, generally any porous material in chopped, mat or woven form, preferably a web of woven glass fibers, is advanced from delivery means 1, which will generally include automatic means for advancing the web at a selected rate and with a selected web tension. The fibrous web is optionally heated by, for example, infrared heaters prior to advancement to the resin application zone. Guiding means 3 is positioned to direct the web toward resin applicator roller 4 at an arc of contact α. Angle α can vary depending upon the overall configuration of the application scheme, but will generally be within the range of 30 to 180 degrees, preferably 90 to 150 degrees, most preferably 120 to 150 degrees to minimize resin waste and maximize heat transfer from roller to substrate. Rotating roller 4 delivers liquid resin film 5 to a first surface of web 2 passing counterdirectional thereto. Applicator roller 4 is maintained at a temperature effective to keep resin film 5 in essentially uncured, liquid form. This temperature will vary depending upon the resin, but will generally be within the range of 50 to 250 °C, preferably 100 to 200 °C. The speed of rotation of applicator roller 4, the tension in web 2 as it contacts resin film 5, and the speed at which web 2 is advanced to the applicator roller are coordinated to provide good wetout of the web. These specifications can vary depending, ■- for example, upon the resin system, the type of web material, and the heating capacity of the downstream B-staging unit. In general, the speed of rotation of applicator roller 4 will be within the range of 70 to 160 percent of web speed, preferably 100 to 140 percent of web speed; the tension in web 2 will generally be within the range of 0.4536 to 1.3608 kg (1 to 3 pounds) per 2.540 cm (linear inch), preferably 0.6804 to 0.9072 kg per 2.540 cm (1.5 to 2 PLI); and the speed of advancement of the web through the resin application zone will be within the range of 0.04064 m/s to 1.016 m/s (8 ft/min to 200 ft/min) , preferably 0.127 m/s to 0.635 m/s (25 to about 125 ft/min). Resin film 5 is applied to applicator roller 4 by means of resin delivery means, shown here as nozzle 7 capable of applying a controlled quantity of liquid resin to the rotating surface of the applicator roller. Nozzle 7 can be associated with any means for continuous delivery of the resin in liquid form, at either ambient or elevated temperature. Delivery of the resin will be carried out at volume rates synchronized with the speed of the moving web so as to deliver a predetermined volume of resin to the web and to minimize residence time within the resin delivery system. Resin delivery means can include, for example, a temperature-controlled static blender, dynamic mixer, or a mixing extruder with an outlet into nozzle 7. Metering means, shown here as a set gap roller 8 located between nozzle 7 and the point of contact of resin film 5 and the advancing web, is used, in conjunction with resin delivery means 7, to control the amount of liquid resin which is delivered to the web. Set gap roller 8 is preferably a smaller-diameter roller than applicator roller 4 rotating countercurrently with respect to the rotation of the applicator roller at a rotation speed within the range of 1 to 50, preferably 8 to 35, percent of the speed of the web. Optional doctor blade 10 is positioned to prevent backflow of resin from the application area, a result also achievable by positioning of the resin deposition area and the set gap roller forward with respect to applicator roll 4, i.e., at an angle β less than 90° with the horizontal radius so that gravitational forces prevent the resin from flowing down the applicator roll in the counterclockwise direction. Control of the rate at which resin is applied to the web is achieved in the first instance by careful setting of the gap between set gap roller 8 and applicator roller 4 so as to maintain a uniform film thickness. Secondly, the rotational speed of the applicator roller is coordinated with web speed so as to achieve transfer of the resin film onto the moving web. In addition, because no backup roll is required to control the contact of the moving web with the applicator roll, control of web tension is maintained to ensure stable operation of the resin application process. Optional resin removal means 9, shown here as a scraper or doctor blade, serves to remove from the applicator roller any resin which remains on the roller after contact between resin film 5 and advancing web 2, prior to passage of this remaining resin through a 360° rotation of the roller. The resin removal means is preferably located with respect to the roller and the advancing porous web so that any resin coming in contact therewith is deposited directly onto the porous web prior to contact of the web with the applicator roller. Resin-containing web 6 is advanced to cure zone 14 via heating zone 11, where the resin-containing web is exposed to a heat source, shown here as a bank of infrared heaters 13. The heat source is most preferably located closely adjacent the resin application area to minimize heat loss from the resin-impregnated web. This intermediate heating step promotes flow of the resin within and through the web, which is also enhanced by advancing the resin-impregnated web to the cure zone at an angle of up to 45°, preferably 20 to 40°, away from vertical. The prepregging process of the invention can be described in general terms by reference to Figure 2. Fibrous web 43 is delivered to resin application zone 44 by a suitable automated web advancement system 42 with means for measuring the controlling advancement speed and web tension. Web tension control devices are known in the art. For example, unwind roll 41 can include a brake which, in combination with a front-end dancer roll, maintains a preset web tension programmed into a pull-in unit located between the dancer roll and the resin application zone. Similarly, proper downstream web tension can be maintained by a dancer roll which moderates the speed of a variable-speed constant-diameter roll located downstream from the heating zone. The fibrous web is advanced through resin application zone 44, which is here shown in a vertical orientation in which the web passes at a controlled rate in a generally upward direction as liquid resin is applied by the method described in detail above. Application zone 44 includes resin delivery means, including a mixer for blending the resin and curing system, temperature control as necessary to maintain the resin system at the desired viscosity, and a heat source as described herein for reduction of heat loss from the resin-impregnated web and improved flow of resin through the web. Optional small-diameter conditioning rollers can be used to promote resin penetration of the web after application and before cure. Resin-saturated web 45 is advanced from the resin application zone to cure zone 46, wherein the resin-saturated web is heated by exposure to a heat source such as a convection oven or infrared heater to partially cure the resin without gelation, a process known as "B-staging". The temperature in the heating zone preceding the cure zone will generally be, within a given system, lower than the temperature maintained in the cure zone, and will usually be at least 100 °C, preferably within the range of 100 to 200 °C, most preferably 130 to 160 °C. (Because of heat loss between the heat source and the resin-containing web, the heat source itself may be operating at temperatures above these given for the heating zone.) The temperature in the cure zone will vary depending upon the resin system and the degree of resin cure desired, but will generally be within the range of 120 to 250 °C, preferably 150 to 230 °C. The resin-saturated web will be subjected to the B-staging treatment for a time sufficient to impart the desired degree of cure,usually up to 15%, preferably up to 10%, more preferably up to 5%, of the curable groups, e.g. epoxy groups, generally 10 seconds to 8 minutes. The web is advanced from resin cure zone 46 in the form of a prepreg 47, which is rolled at uptake roll 48 for storage or, alternatively, is passed directly to lamination. A laminate is fabricated by subjecting a set of layered prepregs to conditions effective to cure the resin and to integrate the prepregs into a laminated structure. The laminate can optionally include one or more layers of a conductive material such as copper. Laminating conditions generally include a time of 30 minutes to 4 hours, preferably 1 hour to 2 hours, a temperature of 160 °C to 300 °C, preferably 170 °C to 200 °C and a pressure of 3.516 to 35.16 kgf/c ^ (50 to 500 psi) . The laminate can optionally be "post-cured" by heating at a temperature of 200 to 230 °C at ambient pressure for 1 to 6 hours to improve thermal properties . An epoxy resin-containing laminating composition will include a curing agent. Effective curing agents for epoxy resins are known to include, for example, amines, acids, anhydrides, phenols and imidazoles. See also EP 476,752. The presently-preferred curing agents for imparting optimum laminating properties to epoxy compositions are phenolic compounds which have a phenolic functionality greater than 1.75. The preferred phenolic curing agents are phenolic novolacs prepared by reacting a dihydroxy phenol such as resorcinol or bisphenol-A with formaldehyde in acid solution. The preferred phenolic novolac resin curing agents are bisphenol-A novolacs having a weight per phenolic group of 60 to 500, preferably 60 to 300, and, on the average, more than 2 phenolic hydroxyl groups per molecule, preferably 3 to 5. Such phenolic novolacs are available under the tradename EPIKURE DX-175 (EPIKURE is a trade mark) from Shell International Chemical Company. The phenolic novolac curing agent will be present in the composition in an amount effective to cure the epoxy resin, which will generally be a stoichiometric amount of 0.75 to 1.25 equivalents per equivalent of epoxy resin. In terms of weight percent, the curing agent will be present in an amount generally from 10 to 70 weight percent, preferably 15 to 50, most preferably 15 to 40, based on the combined weight of epoxy resin and curing agent. The curing agent, for flame-proof applications, can be a mixture of the phenolic resin curing agent and a brominated phenolic curing agent. In order to promote faster and/or lower temperature cure of the resin components of the composition, an optional cure accelerator may be used. The thermosettable resin system must be designed within certain specifications dictated by the resin application process parameters. The resin formulation must be a liquid at a temperature at which the resin does not undergo cure over the time necessary for application of the resin to the substrate. The resin system must be of sufficiently low viscosity that it achieves good "wetout, " or saturation of the web, without the use of a backup roll at the point of application. Once applied to the substrate, however, the resin system must have sufficient viscosity that it does not drop from the resin-containing web before it reaches the heating zone. Resin formulations having viscosities in the range of 0.05 to 1.0 Pa.s (0.5 to 10 poise), preferably 0.05 to 0.6 Pa.s (0.5 to 6 poise), are most suitable. The currently preferred resin system is a blend of a diglycidyl ether of bisphenol-A having a WPE of 175-190, a brominated diglycidyl ether of bisphenol-A having a WPE of 310-350 and a bromine content of 30-50 percent, a phenolic novolac curing agent, and 2-methylimidazole accelerator. The process of the invention can optionally be practiced with a thermosettable resin formulation which includes an organic solvent or diluent present in an amount effective to decrease the viscosity of the system for ease of processing. Polar organic solvents such as ketones, alcohols and glycol ethers, for example, are suitable. The chosen solvent will generally have a boiling point less than 160 °C. The preferred solvents for epoxy resins are ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, for example, and solvent mixtures of these with an alkylene glycol ether such as propylene glycol monomethyl ether. The proportion of solid components in the composition can vary widely depending upon the amount of the other constituents present and the intended application of the composition, but the solvent in a solvent-borne system will generally constitute from 15 to 50 weight percent of the total weight of the formulation. Example 1 An experiment was performed using the invention process to assess the effect of exposing a resin-impregnated substrate to infrared heat between the resin application and partial cure steps of the prepregging process. A solventless resin system based on the diglycidyl ether of bisphenol-A was applied to a glass web 1.27 m (50") in width using an applicator roll configuration generally as shown in Figure 1 having a web angle of contact of 135° and an applicator roll temperature of 127 °C. The opposite side of the resin-impregnated web was passed within 7.62 cm (3 inches) of the centre of a 30.48 cm x 30.48 cm (12"xl2") infrared heater maintained at 370 °C. The centre of the web thus received direct exposure to the infrared heat, while the edges of the web (approximately 45.72 cm (18 inches) on each side) were not directly exposed to the heat. The IR-heated side of the web (across all 1.27 m (50 inches)) was then passed in contact with a 3.81 cm (1.5-inch) (diameter) polishing roll maintained at 149 °C. Inspection of the web found that the area exposed to IR heat glistened, while the edges of the web appeared dull, indicating the desired effect of greater penetration of resin over the IR-exposed area.

Claims: C L A I M S 1. A process for impregnating a porous web with a thermosettable resin by depositing uncured liquid-form thermosettable resin onto a rotating roller, passing the porous web in countercurrent contact with said resin on said roller so as to transfer the thermosettable resin to the porous substrate, and passing the resin-containing porous substrate to a heated cure zone for partial cure of the resin, in which process the resin-containing porous substrate prior to passage to the cure zone is exposed to an elevated temperature greater than 100 °C and less than the highest temperature maintained in said cure zone. 2. A process according to claim 1 in which the elevated temperature is maintained at a temperature within the range of 100 to 200 °C and the cure zone is maintained at a temperature within the range of 120 to 250 °C. 3. A process according to claim 1 or 2 in which the elevated temperature is provided by at least one infrared heater. 4. A process according to claim 1 in which the porous substrate is a glass web. 5. A process according to claim 1 in which the thermosettable resin is an epoxy resin. 6. An apparatus for impregnating a porous web with a thermo¬ settable resin, the apparatus comprising: (a) a rotatable roller; (b) means for depositing a controlled quantity of a liquid-form thermosettable resin onto said rotatable roller; (c) means for advancing the porous web in countercurrent contact with the thermosettable resin on said rotatable roller and then to a heated cure zone; and (d) heating means located between said rotatable roller and said cure zone. 7. An apparatus according to claim 6 in which said heating means is at least one infrared heater. 8. An apparatus according to claim 6 in which said heating means is located closely adjacent said rotatable roller. 9. An apparatus according to claim 6 in which said means for depositing the liquid-form thermosettable resin is a nozzle operatively associated with a dynamic mixer.

Inventors: IYER SHRIDHAR RATNASWAMY, YOUNG GLENDA CAROLE, IYER, SHRIDHAR, RATNASWAMY, YOUNG, GLENDA, CAROLE
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1994
Main classification: B29B15
IPC codes: B05C1, B29B11, B29B15, B29C35, B29C70, H05K3